Major-General Igor Konashenkov of the Russian Defence Ministry said in a statement that the Russian and Syrian air forces did not bomb residential areas and accused the observatory of making up allegations and of sympathising with radical Islamist militants.

Tensions in the Korean Peninsula and a growing crisis over the fate of a nuclear agreement between Iran and world powers has relegated talk of resolving the six-year-old Syrian civil war to back rooms and bilateral chitchat

The document states that the zones “are a temporary measures that will initially last six months and will be automatically extended on the basis of consensus among the guarantor countries”, Efe news reported.

Ceasefires brokered by Russia, Turkey, Iran and the United States in remaining rebel-held areas of western Syria have freed up manpower on the government side, helping its advance east into the oil-rich province of Deir al-Zor.

Jordan said on Monday it was working with Russia to roll out a plan to end fighting in southwestern Syria “in the fastest possible time” – part of a peace pact for the border area brokered by Amman, Moscow and Washington.

The Syrian army this week broke through Islamic State lines to reach a government-held enclave of Deir al-Zor besieged for years by the jihadists, and is fighting to reach a nearby air base which Islamic State still surrounds.

Rapidly losing territory in both Syria and Iraq, Islamic State is falling back on the Euphrates towns downstream of Deir al-Zor, including al-Mayadin and al-Bukamal, where many expect it to make a last stand.